The present invention relates to semiconductor processing, and more particularly to measuring impurity concentrations in susceptors used to secure wafers in epitaxial reactors.
Manufacturers of semiconductor integrated circuits are constantly striving to increase the performance and reduce the price of their products. One way to both increase the performance and reduce the price of an integrated circuit is to reduce the size of the integrated circuit. By reducing the size of a circuit, more circuits can be manufactured on a single semiconductor substrate, thereby reducing the unit cost of the circuit. In addition, reducing the size of a circuit typically increases its speed and reduces its power consumption.
One problem manufacturers encounter in attempting to reduce the size of their integrated circuits involves impurity contamination. For example, metallic contamination of a semiconductor substrate can cause excess leakage currents, poor voltage breakdown characteristics, and reduced minority carrier lifetimes. As the size of an integrated circuit decreases, the detrimental effect of impurities in the semiconductor is magnified. For extremely small circuits, even relatively low levels of contamination can be sufficient to render the circuit inoperative. Therefore, manufacturers take extraordinary measures to prevent impurity contamination in their manufacturing processes.
To optimize their contamination control practices, manufacturers often need to measure the concentration of impurities in their semiconductor substrates at various points during the manufacturing process. This allows manufacturers to determine which area(s) of their process are causing impurity contamination problems. However, as the levels of impurity concentration have decreased to very low levels, it has become more and more difficult to measure the impurity concentration. Indeed, semiconductor industry standards such as the National Semiconductor Roadmap call for impurity concentrations to be as low as 1010 cmxe2x88x923 in the near future. Since the atomic density of a typical semiconductor substrate such as silicon is approximately 1022 cmxe2x88x923, impurity concentrations of 1010 cmxe2x88x923 can be very difficult to measure even with sophisticated measurement equipment.
For example, copper (Cu) and nickel (Ni) are two metallic impurities found in semiconductor substrates. Impurity concentrations of copper and nickel in heavily boron-doped substrates typically are measured by techniques such as Total Reflection X-Ray Fluorescence (TXRF) and Secondary Ion Mass Spectroscopy (SIMS), etc. The minimum detection limit of copper is approximately 1017 cmxe2x88x923 by TXRF (measured near the surface of the substrate) and approximately 1015 cmxe2x88x923 by SIMS. As a result, manufacturers have begun to search for new ways to measure impurity concentrations in semiconductor substrates.
As acceptable levels of metallic impurities are continually being reduced and new methods for measuring impurity concentrations are developed, manufacturers must understand and control the impurity concentrations of equipment used to manufacture semiconductor substrates, and in particular equipment that comes in physical contact with the semiconductor substrate.
One such apparatus of concern is the susceptor used in epitaxial deposition. During epitaxial deposition, the entire backside of the semiconductor substrate is in contact with the susceptor. Since the epitaxial deposition step is performed at relatively high temperatures of approximately 1000xc2x0 C. or higher, any contaminants contained within the susceptor can migrate into the semiconductor wafer, which is very undesirable. It is therefore very important to use equipment that physically contacts the substrate wafer, such as a susceptor, that also has low concentrations of impurities. Unfortunately, reliable methods to determine the concentration of metallic impurities in this type of equipment are destructive. These destructive methods are undesirable because they prevent the ability to ensure that a part is fit for use because the susceptor must be destroyed to obtain the needed results.
The current method to protect from contamination migration is to put a protective layer, such as an oxide layer, on the back of the semiconductor substrate. This, however, is a very expensive process step, and does not add any value to the substrate other than protection. This oxide layer could be eliminated without risk of contamination if a method of determining the impurity concentration level of a susceptor could be achieved. As such, there is a need to be able to non-destructively determine the contamination levels of a susceptor.
The invention provides a method for evaluating the concentration of impurities in an epitaxial susceptor by measuring the concentrations of impurities of a semiconductor wafer that contacts the susceptor. The method includes running an epitaxial cycle with a monitor wafer having contamination levels below detection limits placed on the susceptor and running an epitaxial deposition cycle. At least a portion of the contaminants which have migrated from the susceptor to the monitor wafer are drawn together and measured. In one embodiment of the invention, a gettering layer is formed on the surface of the wafer that was in contact with the susceptor to getter impurities that have migrated from the susceptor. The impurity concentration of the gettering layer is then measured and the resulted are used to determine at least a range of impurity concentrations that were in the susceptor prior to the epitaxial deposition cycle.